Branch Circuit Upstream of Disconnect

[Bestme]

Hello Everyone,
Long time lurker, first time poster here I've got a question for you fine folks regarding a machine design which proposes a branch circuit upstream of the main disconnect.

1. I am reviewing drawings from a machine supplier from Italy for a new project in the US
2. The machine will be fed with a 480 drop
3. BEFORE the disconnect and lockout means, they have branched off a circuit to power a single leg 220:24vdc power supply
4. This supply is used to monitor safety and door switches even with the machine disconnect in the off position
5. The 24vdc is rated at 2 amps
6. They have signs on the backplane, but I've never been comfortable with any power staying live in an enclosure after the main disconnect is thrown. Does this violate anything related to OSHA or NFPA? They assure me it is UL compliant.

If there aren't any specific rules broken by this, I may be okay with the safety circuit as is if they provide a local flip type circuit breaker on the 220 feed line. I'd anticipate this being used if the maintenance team ever needs to replace the power supply. It's a "touch safe" power supply with the feed terminals totally covered by plastic guarding.

My other thought would be to have a disconnect on the drop where the maintenance team can get to it easily. Typically we drop power from above (where it could disconnected at the breaker panel) to the OEM's equipment below on the line. We don't often have a disconnect between our drop and the OEM's equipment disconnect.

Thanks for the help!

 

[jim dungar]

1. They have signs on the backplane, but I've never been comfortable with any power staying live in an enclosure after the main disconnect is thrown. Does this violate anything related to OSHA or NFPA? They assure me it is UL compliant.

It is allowable.
I believe there should be signage, advising two sources of power, before the equipment door is opened.

A good place to check would be NFPA79 and UL508A both of which address control panels for industrial machinery.

 

[Bestme]

It is allowable.
I believe there should be signage, advising two sources of power, before the equipment door is opened.

A good place to check would be NFPA79 and UL508A both of which address control panels for industrial machinery.

Thanks!

 

[controlled]

I do it all the time when wiring machines/panels for automotive plants.
They typically keep just the PLC processor powered. We would use wiring that had an orange tracer stripe on the wire, as well as run the wiring in orange non metalic liquid tight sleeve throughout the panel

 

[Jraef]

NFPA 79 (machinery control requirements) requires that conductors of circuits that are NOT disconnected with the main power disconnect be identified by using yellow or orange wire, and any grounded conductors of those circuits to be white with yellow or orange stripes.

 

[Bestme]

Thanks everyone for your replies. One more question to this point: How does this comply with OSHA 1910.147(c)(4)(i). My interpretation of that says there should be a single lockout point for working on an enclosure and that all "hazardous energy" is controlled. OSHA is always the most difficult one for me to understand because it is so dated. Any help?

 

[Jraef]

From OSHA's standpoint, working on the MACHINE would be fine, because the energy coming off of that power supply is not "hazardous" and cannot cause the machine to operate.

But to work on the PANEL, then OSHA would require that the energy be disconnected AHEAD of that panel, like at the feeder. If the feeder is not lockable, then you would need another disconnect that kills ALL power to the panel. As a general rule, it's best (and simpler) to have ONE disconnect means that does both, but, especially with machine safety systems, that's not always practical.